In many fuel ignition systems, high voltage spark generators are used to ignite the fuel. This is particularly true in systems that utilize fuel oil and gas for various heating applications. In these systems, it is necessary to provide for a spark gap for ignition, and also provide some means of monitoring the existence of the flame. In most conventional systems this is accomplished by providing one pair of electrodes for the spark ignition source and the use of an entirely separate means for flame sensing. The flame sensing can be done by detecting the ultraviolet radiation from the flame, the visible light radiated from the flame, the infrared radiation from the flame, or by the conductivity of the flame itself. In many of the simpler systems, flame conductivity or flame rectification have been used as a means of sensing the presence of the flame.
In most prior systems that use spark ignition for lighting the fuel initially, a separate pair of electrodes are normally used if flame rectification or flame conductivity are used as a flame sensing means. An example of such an arrangement is disclosed in the U.S. Pat. No. 3,619,097 which utilizes a pair of electrodes connected to a relaxation type of silicon controlled oscillator that periodically discharges a capacitor across the pair of electrodes, and this system uses a further sensing electrode to sense the flame by rectification principles between the sensor and the burner structure itself. The use of two separate circuits adds unnecessarily to the expense and the complications of installing a system.